This is simple and complicated at the same time.
A healthy relationship is where both parties needs are being met.
However, people change, grow, mature, needs change, wants change, and these things are not always compatible with the person you have chosen to be with.

I don't believe humans are designed to be with the same person for all time, there are just too many variables.

This doesn't mean unhappiness and grief must ensue, it is possible to accept a relationship has run it's course, grieve it's loss and move on.